Since space technology has been mastered by mankind in 1950s, Europe and America have conducted a series of research on extraterrestrial planets including the Moon, Mars, Venus and satellites of Jupiter. However, information on geological structure and distribution of regolith thickness of the extraterrestrial planets is still poorly understood. More advanced detection device is required to explore the geological structure of regolith or rocks on the extraterrestrial planets, to study topographies of the extraterrestrial planets, to estimate the content of mineral resources, and to provide a scientific basis for researching the formation process and evolution history of the extraterrestrial planets.
To study the geological structure of the Moon, Apollo Lunar Sounder Experiment (ALSE) detection radar, a multi-band penetrating detection radar, has been carried by the Apollo-17 spacecraft in 1973. The ALSE radar include three detection bands which are HF1 band, HF2 band and VHF band respectively. The ALSE radar has completed a total 13 hours exploration and obtained the geological structure data of a depth of 1˜2 km below the surface of the Moon. However, due to the limitations in electronic technology at that time, the radar has several shortcomings, mainly in pool signal quality, low signal-to-noise ratio, no phase information, narrow bandwidth, low depth resolution and short detection time and etc. For example, the depth resolution of the radar is about 150 m, which cannot distinguish the thickness distribution of the lunar regolith. In addition, since the radar data have been recorded on optical films, specific equipment developed by NASA is needed to read the data, which causing extreme difficulty in subsequent data processing. Till now, only a part of results from the shadow areas on the Moon have been published by NASA. From the published data, although ALSE radar is poor at discerning the composition and thickness of geological layers, these experiment results still provide important information for studying the Moon.
Japan developed a lunar spacecraft in the 1990s named Kaguya (SELENE), which carried the Lunar radar Sounder (LRS) used for detecting geological structure at depths of 4-5 km under the surface of the Moon. SELENE-LRS radar operated in a frequency range of 4-6 MHz, and transmitted chirp signal with a pulse width of 200 μs and pulse power of 800 W. The power consumption and weight were respectively 50 W and 24 kg. The antenna system consists of two dipole antennas crossed each other. One antenna is used to transmit an electromagnetic pulse signal, and both antennas are used to receive the echo signals from underground of the moon. SELENE-LRS is mainly utilized to detect geological structure that 1-5 km below the surface of the Moon. SELENE was launched on Sep. 14, 2007 with an orbital height of 100 km from the Moon. From Nov. 20, 2007, LRS radar began to work. The LRS radar system obtained data of the geological structure from hundreds of meters to kilometers below the lunar surface, but the resolution of geological layer was poor.
To detect geological structure of sub-surface layer under Mars and to detect whether there is water-ice structure in the surface on the Mars, European space agency launched Mars Express Orbiter in 2004 which carried MARSIS radar system. The MARSIS radar system worked at low frequency band with a central frequency of 1.5-5.5 MHz, and had narrow band and poor resolution. In order to compensate the resolution of the MARSIS radar system on detecting geological structure of a sub-surface layer, NASA launched the Mars Reconnaissance Orbiter (MRO) in August, 2005. The MRO carried a radar system named SHARAD which is similar to MARSIS radar system. However, since the SHARAD system had a higher frequency band (20 MHz) and a wider bandwidth (10 MHz), its resolution on detecting the geological structure of Mars is much higher than the MARSIS system.
However, in the procedure of practicing the disclosure, the applicant realizes that the ALSE radar, the MARSIS radar and the SHARAD radar described above all use a duplex antenna which has only one receiving channel, while the LRS radar uses a cross-polar antenna. These radars need to assume the dielectric constant previously to calculate the thickness of the sub-surface layer of the Moon and Mars, resulting in a large error. Further, these radar systems are all installed on an orbiter sent to the Moon or Mars, due to the limitation of the installation space and detection technology, the radars have narrow band and low detection resolution which are not sufficient to detect the geological structure and the thickness distribution of regolith on the Moon or Mars.